1. Technical Field
The present invention relates to control devices for a four-wheel drive vehicle to appropriately distribute driving force generated by a driving source such as an engine based on forward information obtained by a camera system and a navigation system.
2. Related Art
Various kinds of control devices for four-wheel drive vehicles have been suggested and put into practice. and, for instance, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-280776 discloses a control device for a four-wheel drive vehicle including a rear-wheel driving system in which rear wheels serves as driving wheels in a two-wheel drive mode as well as a four-wheel drive mode, a force transmission switching mechanism for disengaging or engaging a front-wheel driving system in which front wheels serves as non-driving wheels in the two-wheel drive mode, and a free running differential mechanism for engaging or disengaging the front-wheel driving system at a predetermined position. Neither of the mechanisms includes any synchronization device, but the controller includes a control unit to control both of the mechanisms as follows: in the four-wheel drive mode, the rear-wheel driving system and the front-wheel driving system are engaged and the front-wheel driving system is engaged at the predetermined position. In the two-wheel drive mode from when an ignition switch is turned on to when the four-wheel drive mode is first established, the rear-wheel driving system and the front-wheel driving system are disengaged and the front-wheel driving system is disengaged at the predetermined position. In the two-wheel drive mode from when four-wheel drive mode is once established to when the ignition switch is turned off, the rear-wheel driving system and the front-wheel driving system are disengaged and the front-wheel driving system is engaged at the predetermined position.
In a typical four-wheel drive vehicle, when unnecessary rotation of a drive shaft is stopped during the two-wheel drive mode in order to reduce driving resistance, rotation synchronization is required for switching the two-wheel drive mode to the four-wheel drive mode. The control device for a four-wheel drive vehicle disclosed in JP-A No. 2000-080776 described above address this problem upon switching between the two-wheel drive mode and the four-wheel drive mode. However, with this controller, it is impossible to stop unnecessary rotation of the drive shaft even when the four-wheel drive mode is once returned back to the two-wheel drive mode in order to reduce the driving resistance during driving, and the effect of reducing the driving resistance is impaired. Even when some rotation synchronization function is used, it takes a considerable time to perform switching to the four-wheel drive mode, the differential lock thereof, or the driving force distribution control, which may cause a delay in operation.